三元运算符是两倍慢作为的if-else块？ [英] Ternary operator is twice as slow as an if-else block?
问题描述
我看了到处是三元运营商应该是比速度，或至少一样，其等效如果 - 其他块。
I read everywhere that ternary operator is supposed to be faster than, or at least the same as, its equivalent if-else block.
不过，我做了以下的测试，发现它并非如此：
However, I did the following test and found out it's not the case:
Random r = new Random();
int[] array = new int[20000000];
for(int i = 0; i < array.Length; i++)
{
array[i] = r.Next(int.MinValue, int.MaxValue);
}
Array.Sort(array);
long value = 0;
DateTime begin = DateTime.UtcNow;
foreach (int i in array)
{
if (i > 0)
{
value += 2;
}
else
{
value += 3;
}
// if-else block above takes on average 85 ms
// OR I can use a ternary operator:
// value += i > 0 ? 2 : 3; // takes 157 ms
}
DateTime end = DateTime.UtcNow;
MessageBox.Show("Measured time: " + (end-begin).TotalMilliseconds + " ms.\r\nResult = " + value.ToString());
我的电脑拿了85毫秒运行上面的code。但是，如果我注释掉如果 - 其他块，并取消对三元运算符线，大约需要157毫秒
My computer took 85 ms to run the code above. But if I comment out the if-else chunk, and uncomment the ternary operator line, it will take about 157 ms.
为什么会出现这种情况？
Why is this happening?
推荐答案
要回答这个问题，我们将研究由X86和X64即时编译器为每种情况生成的汇编code。
To answer this question, we'll examine the assembly code produced by the X86 and X64 JITs for each of these cases.
32: foreach (int i in array)
0000007c 33 D2 xor edx,edx
0000007e 83 7E 04 00 cmp dword ptr [esi+4],0
00000082 7E 1C jle 000000A0
00000084 8B 44 96 08 mov eax,dword ptr [esi+edx*4+8]
33: {
34: if (i > 0)
00000088 85 C0 test eax,eax
0000008a 7E 08 jle 00000094
35: {
36: value += 2;
0000008c 83 C3 02 add ebx,2
0000008f 83 D7 00 adc edi,0
00000092 EB 06 jmp 0000009A
37: }
38: else
39: {
40: value += 3;
00000094 83 C3 03 add ebx,3
00000097 83 D7 00 adc edi,0
0000009a 42 inc edx
32: foreach (int i in array)
0000009b 39 56 04 cmp dword ptr [esi+4],edx
0000009e 7F E4 jg 00000084
30: for (int x = 0; x < iterations; x++)
000000a0 41 inc ecx
000000a1 3B 4D F0 cmp ecx,dword ptr [ebp-10h]
000000a4 7C D6 jl 0000007C
X86，三元
59: foreach (int i in array)
00000075 33 F6 xor esi,esi
00000077 83 7F 04 00 cmp dword ptr [edi+4],0
0000007b 7E 2D jle 000000AA
0000007d 8B 44 B7 08 mov eax,dword ptr [edi+esi*4+8]
60: {
61: value += i > 0 ? 2 : 3;
00000081 85 C0 test eax,eax
00000083 7F 07 jg 0000008C
00000085 BA 03 00 00 00 mov edx,3
0000008a EB 05 jmp 00000091
0000008c BA 02 00 00 00 mov edx,2
00000091 8B C3 mov eax,ebx
00000093 8B 4D EC mov ecx,dword ptr [ebp-14h]
00000096 8B DA mov ebx,edx
00000098 C1 FB 1F sar ebx,1Fh
0000009b 03 C2 add eax,edx
0000009d 13 CB adc ecx,ebx
0000009f 89 4D EC mov dword ptr [ebp-14h],ecx
000000a2 8B D8 mov ebx,eax
000000a4 46 inc esi
59: foreach (int i in array)
000000a5 39 77 04 cmp dword ptr [edi+4],esi
000000a8 7F D3 jg 0000007D
57: for (int x = 0; x < iterations; x++)
000000aa FF 45 E4 inc dword ptr [ebp-1Ch]
000000ad 8B 45 E4 mov eax,dword ptr [ebp-1Ch]
000000b0 3B 45 F0 cmp eax,dword ptr [ebp-10h]
000000b3 7C C0 jl 00000075
X64，如果/然后
32: foreach (int i in array)
00000059 4C 8B 4F 08 mov r9,qword ptr [rdi+8]
0000005d 0F 1F 00 nop dword ptr [rax]
00000060 45 85 C9 test r9d,r9d
00000063 7E 2B jle 0000000000000090
00000065 33 D2 xor edx,edx
00000067 45 33 C0 xor r8d,r8d
0000006a 4C 8B 57 08 mov r10,qword ptr [rdi+8]
0000006e 66 90 xchg ax,ax
00000070 42 8B 44 07 10 mov eax,dword ptr [rdi+r8+10h]
33: {
34: if (i > 0)
00000075 85 C0 test eax,eax
00000077 7E 07 jle 0000000000000080
35: {
36: value += 2;
00000079 48 83 C5 02 add rbp,2
0000007d EB 05 jmp 0000000000000084
0000007f 90 nop
37: }
38: else
39: {
40: value += 3;
00000080 48 83 C5 03 add rbp,3
00000084 FF C2 inc edx
00000086 49 83 C0 04 add r8,4
32: foreach (int i in array)
0000008a 41 3B D2 cmp edx,r10d
0000008d 7C E1 jl 0000000000000070
0000008f 90 nop
30: for (int x = 0; x < iterations; x++)
00000090 FF C1 inc ecx
00000092 41 3B CC cmp ecx,r12d
00000095 7C C9 jl 0000000000000060
X64，三元
59: foreach (int i in array)
00000044 4C 8B 4F 08 mov r9,qword ptr [rdi+8]
00000048 45 85 C9 test r9d,r9d
0000004b 7E 2F jle 000000000000007C
0000004d 45 33 C0 xor r8d,r8d
00000050 33 D2 xor edx,edx
00000052 4C 8B 57 08 mov r10,qword ptr [rdi+8]
00000056 8B 44 17 10 mov eax,dword ptr [rdi+rdx+10h]
60: {
61: value += i > 0 ? 2 : 3;
0000005a 85 C0 test eax,eax
0000005c 7F 07 jg 0000000000000065
0000005e B8 03 00 00 00 mov eax,3
00000063 EB 05 jmp 000000000000006A
00000065 B8 02 00 00 00 mov eax,2
0000006a 48 63 C0 movsxd rax,eax
0000006d 4C 03 E0 add r12,rax
00000070 41 FF C0 inc r8d
00000073 48 83 C2 04 add rdx,4
59: foreach (int i in array)
00000077 45 3B C2 cmp r8d,r10d
0000007a 7C DA jl 0000000000000056
57: for (int x = 0; x < iterations; x++)
0000007c FF C1 inc ecx
0000007e 3B CD cmp ecx,ebp
00000080 7C C6 jl 0000000000000048
第一：为什么是X86 code的这么多的X64相比慢
First: why is the X86 code so much slower than X64?
这是由于code的下列特性：
This is due to the following characteristics of the code:
- X64有几个可用的额外的寄存器，每个寄存器为64位。这使得X64 JIT完全通过寄存器除了加载执行内环
I从数组，而X86 JIT放置在回路数栈操作（内存访问）。 -
值是一个64位整数，这需要随后<$上X86（添加2的机器指令C $ C> ADC ），但只有在X64（添加）。 1
- X64 has several additional registers available, and each register is 64-bits. This allows the X64 JIT to perform the inner loop entirely using registers aside from loading
ifrom the array, while the X86 JIT places several stack operations (memory access) in the loop. valueis a 64-bit integer, which requires 2 machine instructions on X86 (addfollowed byadc) but only 1 on X64 (add).
二：为什么是三元运算符x86和X64慢
这是由于影响了JIT的优化操作的顺序一个微妙的差异。以JIT三元运算符，而不是直接编码 2 和 3 在添加机器指令自己，JIT创建一个中间变量（在一个寄存器）保存结果。该寄存器，然后从32位符号扩展到64位将其添加到值之前。因为所有这一切都是在X64寄存器尽管复杂三元运营商的净影响在一定程度最小化显著上升执行。
This is due to a subtle difference in the order of operations impacting the JIT's optimizer. To JIT the ternary operator, rather than directly coding 2 and 3 in the add machine instructions themselves, the JIT creating an intermediate variable (in a register) to hold the result. This register is then sign-extended from 32-bits to 64-bits before adding it to value. Since all of this is performed in registers for X64, despite the significant increase in complexity for the ternary operator the net impact is somewhat minimized.
另一方面的X86 JIT被撞击在更大程度上，因为除了在内环一个新的中间值的导致其溢出另一值，导致至少2个额外的存储器访问中的内环（看到访问 [EBP-14H] 在X86三元code）。
The X86 JIT on the other hand is impacted to a greater extent because the addition of a new intermediate value in the inner loop causes it to "spill" another value, resulting in at least 2 additional memory accesses in the inner loop (see the accesses to [ebp-14h] in the X86 ternary code).
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